//#include <iostream>
//using namespace std;
//int main()
//{
//	char a, b, c, d, e;
//	cin >> a >> b >> c >> d >> e;
//	cout << e << d << c << b << a;
//	return 0;
//}


//#include<iostream>
//#include<cstdio>
//#include<cmath>
//using namespace std;
//int main()
//{
//	double a, b, c;
//	cin >> a >> b >> c;
//	double p = (a + b + c) / 2;
//	double S = sqrt(p * (p - a) * (p - b) * (p - c));
//	printf("%.1f",S);
//	return 0;
//}


//#include<iostream>
//#include<cstdio>
//#include<cmath>
//#include<iomanip> 
//using namespace std;
//int main()
//{
//	double a, b, c;
//	cin >> a >> b >> c;
//	double p = (a + b + c) / 2;
//	double S = sqrt(p * (p - a) * (p - b) * (p - c));
//	cout << fixed << setprecision(1) << S << endl;
//	return 0;
//}
